A thermal transfer printer which is configured to include a print-correction feature is often found on typewriters. This correction feature allows the removal of unintentionally printed characters.
Thermal transfer printers are generally made up of a printing surface, or platen, an ink ribbon, a thermal head and a printing medium (normally a sheet of paper). The ink ribbon is comprised of a base ribbon upon one side of which is deposited an ink film. During the printing operation, the ink ribbon is disposed between the paper, the thermal head and the platen such that the base ribbon is facing the thermal head, the ink film is facing the paper, and the platen is located on the side of the paper opposite the ink ribbon. When the positioning mechanism of the thermal head is activated during the printing process to move toward the ink ribbon, the ink ribbon and paper are pressed together between the platen and the thermal head. When heat in a predetermined pattern is applied to the ink ribbon from the heating elements on the thermal head, the heat travels through the base ribbon and melts the ink located directly across from the activated heating elements. The molten ink then separates from the ink ribbon and is held between the ink ribbon and the paper. When the heat from the thermal head is removed, the ink cools and solidifies. Because the ink has a greater affinity for the paper than for the ink ribbon, the ink adheres to the paper in the predetermined pattern produced by the heating elements on the thermal head.
Early developed methods to remove a character printed by a thermal printer involve a process similar to the printing described above; however, instead of an ink ribbon, a correction ribbon is used. The thermal head is positioned over a character to be removed and the character is heated. Because the ink has a greater affinity for the correction ribbon than for the paper, the ink adheres to the correction ribbon when the correction ribbon is peeled away from the sheet of paper. The problem with this method, however, is that if the ink does not sufficiently cool and solidify before the correction ribbon is peeled away from the sheet of paper, at least a portion of the ink will be left on the paper.
The problem associated with the method of correcting thermal transfer printing described above is addressed in the prior art by the introduction of a correction pin located next to the thermal head and on the downstream side of the correction ribbon. When a correction was effected, the correction pin extends to press the correction ribbon against the paper such that when the thermal head is moved away from the position in front of the corrected character, the ribbon remains pressed against the paper, allowing the ink to cool and solidify. Once the ink cools and adheres to the correction ribbon, the correction pin is removed and the correction ribbon is peeled away from the paper, thereby removing the character from the paper.
A problem still remains with the correction pin method of ink removal. When the ink cools and solidifies, it adheres to both the paper and the correction ribbon. Because the ink has a stronger affinity for the correction ribbon than for the paper, when the correction ribbon is peeled away with sufficient force from the paper, the ink is separated from the paper. However, if the force peeling the ribbon away from the paper is not sufficient, the ink acts as a glue and the ribbon becomes adhered to the paper. When printing or correcting process is continued, the correction ribbon becomes fouled and damage to the ribbon results.
Ink and correction ribbons are usually enclosed within a ribbon cassette which provides both protection for the ribbons and a convenient method of replacing or exchanging ribbons on the thermal transfer printer. The cassettes are generally made up of a housing in the shape of a thin rectangular box. The ribbon is unwound from a supply reel and, as it is used, is rewound onto a take-up reel. The exposed portion of the ribbon, located between the supply reel and the take-up reel, is positioned by the cassette housing such that when the cassette is connected to the thermal transfer printer, the exposed portion is disposed between the printer head and the paper.
The method of transferring ribbon from the supply reel to the thermal head and finally to the take-up reel involves the use of a motor connected to the take-up reel. When the motor is activated, force is applied to the take-up reel such that the ribbon is drawn to and wound around the take-up reel. This action results in tensile force along the exposed portion of the ribbon and also causes the ribbon to unwind from the supply reel.
The prior art correction pin method relied upon the motor driving the take-up reel to provide the necessary force to peel the correction ribbon containing the ink from the paper when the correction pin was withdrawn. The problem associated with this prior art method was that the tensile force exerted on the ribbon varied with the amount of ribbon already removed onto the take-up reel. As the take-up reel accumulated ribbon, the amount of tensile force exerted on the ribbon decreases. This decrease in tensile force can result in insufficient force to peel the ink from the paper and ribbon fouling can result.
In addition to correction operations involving black ink, certain thermal transfer printers capable of printing in different colors from a single ribbon are benefitted by this constant tension device. Because printing in different colors on these thermal transfer printers involves detecting the color of the ink in front of the print head, it is important to keep the color ink ribbon taut and moving at a constant rate at the time of detection. In these color thermal transfer printers, the method of providing the tension along the color ink ribbon and controlling the ribbon feed rate is similar to the black-ink-ribbon thermal transfer printers; that is, the take-up reel is provided a nonvarying torque to draw the ribbon from the supply reel. The varying tension and feed rate caused by various amounts of color ink ribbon on the take-up reel make color detection unreliable.